Thursday, 5 March 2015

Thick Dust Plumes Obscure Africa’s Coast

Hundreds of millions of tons of sand and dust particles are lifted from North African deserts each year and carried across the Atlantic Ocean. So much dust is kicked up that the microscopic particles amass into sweeping tan plumes that are visible to satellites.

On February 26, 2015, the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua
satellite observed some of that dust starting a trans-Atlantic journey.
In the image above, vast amounts of dust rise up from Senegal,
Mauritania, and Gambia. The plumes are thick and brown, suggesting that
he dust is still compact and that it probably arose close to the
coast—not from a more distant location in the North African interior.
Some of the dust also appears to be settling into the waters just
offshore, adding to the darkening effect in the satellite view. A bit
farther offshore, the water surface is brightened by sunglint, the
reflection of sunlight directly back at the camera from a relatively
smooth surface.

By the next day, the MODIS instrument on NASA’s Terra
satellite observed the dust passing over the Cabo Verde Islands
(below). The dust cloud was lighter in color, as the coarser, larger
sand grains likely dropped out closer to the African coast. Note how the
volcanic islands created wake patterns in the dust, as the rough
mountain peaks stuck out from the smooth ocean surface and changed the
air flow on the leeward side. Low-level dust was probably deposited on
the islands, as well.

Note also the semi-circular pattern of the clouds to the west, with a
region of clear, dry air between the clouds and the dust storm. The
pattern was likely caused by air advecting from the North African
desert; a front of dry air in a high-pressure weather system prevented
cloud formation ahead of the dust.
By the time the Visible Infrared Imaging Radiometer Suite on the Suomi NPP
satellite got a look on February 28 (below), the clear, dry air had
been overtaken by the dust, which was half-way to South America. Note,
too, the faint gray pall of smoke from fires farther down the African
coast.

In a new paper published on February 24, 2015, scientists using a
NASA satellite announced that they had quantified in three dimensions
how much dust makes the trans-Atlantic journey from the Sahara Desert to
South America. Scientists not only measured the volume of dust, but
they also calculated how much phosphorus—remnant in Saharan sands from
the desert’s ancient past as a lake bed—gets carried from one of the
planet’s most desolate places to one of its most fertile. The work by Yu and colleagues appeared in the journal Geophysical Research Letters.

The trans-continental journey of dust is important because of what is
often in the dust, said lead author Hongbin Yu, an atmospheric
scientist from the University of Maryland who is based at NASA’s Goddard
Space Flight Center. Dust picked up from the Bodélé Depression in
Chad—farther inland than in the storm shown above—has rock minerals
loaded with phosphorus, an essential nutrient for plant proteins and
growth. The Amazon rainforest depends on this dust to flourish because
soil nutrients are in short supply in Amazonian soils.

Fallen, decomposing leaves and organic matter provide the majority of
nutrients in the rainforest, but those nutrients are rapidly absorbed
by plants and trees after entering the soil. But some nutrients like
phosphorus are washed away by rainfall into streams and rivers, draining
from the Amazon basin like a slowly leaking bathtub. The phosphorus
that reaches Amazon soils from Saharan dust—an estimated 22,000 tons per
year—is about the same amount as that lost from rain and flooding, Yu
said.

The new dust transport estimates were derived from data collected
from 2007 though 2013 by NASA’s Cloud-Aerosol Lidar and Infrared
Pathfinder Satellite Observation, or CALIPSO. The data show that wind
and weather pick up on average 182 million tons of dust each year and
carry it past the western edge of the Sahara. The dust then travels
1,600 miles across the Atlantic Ocean, though some drops to the surface
or is flushed from the sky by rain. Near the eastern coast of South
America, 132 million tons remain in the air and 27.7 million tons fall
to the surface over the Amazon basin. About 43 million tons of dust
travel farther to settle out over the Caribbean Sea.